Such drift chambers consist conventionally of a linear array of conducting electrodes, which may take various forms, electrically insulated from one another, and held at various suitable electrical potentials in order to create appropriate electric fields in the spaces between them, usually along the axis of the tube.
The electrode array is normally enclosed in a cylindrical gas-tight chamber furnished with openings through which sample vapours or gases, and circulating carrier gas, may be introduced.
Ion mobility spectrometers may be used to detect the presence in an ambient atmosphere of gases or vapours arising from of substances of interest e.g explosives, drugs, and organic pollutants of various types.
An ion mobility spectrometer (IMS) will typically comprise an ionization source, a reaction region, a drift tube, a gating, or injection, grid between the reaction region and the drift space, and an ion detector, coupled to a collector electrode within the drift chamber.
The modus operandi of ion mobility spectrometers is well known. Briefly, a carrier gas, typically dry air, is introduced into the spectrometer together with the sample gas or vapour, and is fed via an inlet to the reaction region containing an ionization source such as nickel 63 or a corona ionization element, resulting in a partial ionisation of the molecules of the carrier gas and the sample. Additional charge may also be transferred by impact from carrier gas molecules to sample molecules.
Within the reaction region a potential gradient is usually present, moving the charged mixture of sample and carrier molecules towards the injection grid. The grid is held at a potential such as to block transfer of the charged mixture to the drift chamber except when the potential is periodically reduced, thereby permitting a package or "pulse" of ions to enter the drift chamber.
Within the drift chamber an approximately constant potential gradient, arising as a result of the potentials is applied to the successive electrodes of the electrode array, moves the injected ions towards a collector electrode, located at the end of the drift chamber remote from the reaction region, where the ion charges are collected.
The time of arrival of the ions with respect to the opening of the injection grid is dependent on the mobility of the ions, light ions reaching the collector electrode sooner than heavier ones. The identity of the ions, and hence of the original molecules and of the substances, may be established by reference to the time of flight within the drift chamber, and the relative concentration of the ions, and hence of the molecules and of the substances, by reference to the magnitude of the respective collector currents. The opening of the injection grid is usually made periodic to increase the signal-to-noise ratio of the system, or in order to perform a continuing series of measurements.
To realise an IMS drift chamber the various electrodes must be supported in their correct relative positions, insulated from one another, and supplied with the appropriate voltages.
In the past various different constructional methods have been adopted. However many have involved a "stack" of annular metallic electrodes separated by insulators. Such a "stack" may be threaded on to two or more columns, or, in some cases, the insulators may be configured to locate the electrodes and the whole stack placed under axial compression.
The cell contained within such a stack may be hermetically sealed from the surrounding atmosphere by nature of its construction e.g by the use of compressible insulators between the electrodes.
In general each electrode will need to be held at a unique electrical potential; in some cases this potential will be time-variable to provide a gating function. It is most convenient if each electrode is supplied with its own connection to circuitry outside the containment, but this will usually involve numerous gas-tight lead-throughs for the drift chamber, at least some of which will need to operate at high potential.
Ion mobility spectrometers using drift tubes such as described above tend to be relatively expensive to build because of the need for precision machined components, the complex assembly operations involved, and the multiplicity of electrical connections.